Many industrial processes utilize aqueous solutions and suspensions in which foam production is detrimental to the efficient conduct of the process. Traditional defoaming agents which are commonly employed control foam during these processes but may lead to insoluble material deposition upon process equipment, requiring costly and inconvenient clean up and down time. Examples of processing in which foam is detrimental, and insoluble material deposition is commonly found includes, but is not limited to, paper manufacturing, paper de-inking, textile processing and sewage disposal systems. The insoluble material used for de-foaming may actually deposit upon the surface of the substrate being processed. If this occurs, the substrate will be of inferior quality and may have to be reprocessed to remove insoluble material.
In the paper industry the Kraft process is one of the most frequently used alkaline pulping processes. It is valuable in that spent chemicals may be recycled and reused thus decreasing processing costs. A large disadvantage of this process is the occurrence of foam during the pulp screening and washing procedures.
The Kraft process as taught in U.S. Pat. No. 3,215,635 to Liebling is generally described as being performed by first cooking the wood chips in digesters and then drawing off the spent chemicals for reuse. The resulting pulp fibers are then washed free in brown stock washers of a large amount of residual chemicals. These washers are a series of vats usually three or four in number which alternatively dilute the pulp with water and thicken it by picking it up on large rotary screens. From the brown stock washers, the pulp travels to the screen room where it is again diluted with water and put through vibrating screens which accept the now completely delignified fibers and reject the clumps of unpulped fibers, knots and other foreign material. Foam problems are severe in the screen room since the diluted pulp is subjected to violent agitation by the screens. The water removed from the pulp after the screening operations is referred to as the dilute black liquor and, for the sake of economy, is normally used as the dilution water for the third and fourth stage of the brown stock washers. The dilute black liquor is a foaming material, containing from about 0.001% to 0.1% by weight of solids and has a pH of about 12. The foaming of the dilute black liquor increases with the increase of the resin content of the wood used in this process.
Defoamers are generally used in most alkaline pulp mills during the screening operations so that a more efficient screening is accomplished and to prevent the pulp thickeners, utilized after the screening operations, from becoming clogged with entrapped air. When water dispersible defoamers are used during the screening operation, the control of foam and entrained air in the screening operation contributes to the washing efficiency of the pulp during the alkaline pulping process. This is accomplished by the fact that the screening efficiency of the pulp is increased, allowing ease of flow of the pulp throughout the thickeners and subsequent washers.
Additionally the paper de-inking process uses detergents which can cause considerable undesired foam when performing the desired removal of ink. More detailed descriptions of such processes are found in standard textbooks, such as A. M. Schwartz and J. W. Perry "Surface Active Agents", Vol. I (1949); and "Surface Active Agents and Detergents" Vol. II (1958). Interscience Publishers, New York, the descriptions of which are incorporated herein by reference.
The de-inking agents are used in solution in substantially aqueous media. The temperature of the de-inking solution may vary anywhere from room temperature, e.g., 40.degree.-70.degree. F. (4.degree.-20.degree. C.), up to about 200.degree. F. (95.degree. C.). Best results are achieved with the de-inking solutions described herein when they are alkaline in pH. It therefore is desirable that an alkali be included therein. Although any suitable alkali or alkaline earth metal hydroxide or salt may be employed, the alkali metal hydroxides and salts, such as sodium hydroxide, potassium hydroxide, soda ash and the like are preferred. Enough of the alkali should be added to maintain the pH of the de-inking solution between about 7.0 and 11.5, or even higher, and preferably at least 7.1.
In preparing the de-inking solution, water is charged to the reactor or pulper and about 0.3 to 3 percent, based on the weight of the paper, of the de-inking agent described hereinabove is added. The de-inking agent is preferably added to the water prior to the addition of the wastepaper or waste.
To the resulting solution is added the printed paper, scrap or waste. The printed cellulosic charge may, if desired, be shredded by appropriate means prior to treatment. This, however, is not necessary, and the waste material may be added to the treating solution without shredding or without any subdivision in size whatsoever. It is one of the advantages of this invention that costly shredding or pulping techniques prior to de-inking need not be employed. Thus, the waste material to be de-inked is preferably added to the testing solution in its naturally dry condition, i.e., without being subjected to mositure or water other than that which is normally present in the atmosphere. Although de-inking will occur if the waste material is first slurred or pulped in water, in general it has been found that the results achieved are inferior to those obtained when the waste material is added to the treating solution in its naturally dry condition, i.e., in equilibrium with its natural atmospheric environment. Although not wanting to be restricted to this interpretation, it appears that wetting the waste material with water prior to subjecting it to the chemical treatment described herein has a tendency to set the ink and make it more difficult to remove from the cellulosic fibers. The amount of the scrap or waste added to the treating solution should be controlled. In general, the per cent of cellulosic by weight of the aqueous de-inking solution should be below 10 per cent and preferably below 6.0 per cent, or between about 4.0 and 6.0 percent. Good results are obtained when the de-inking solution contains about 5 to 5.5 per cent lb. weight of paper and this value appears to be optimum. The scrap is retained in the treating solution until substantial defiberization and separation of the ink takes place.
Following treatment, the defibered material is dropped to a chest or other reservoir, after which it is diluted with water to a solids content of between about 0.5 and 1.5 percent, preferably about 1.0 percent, based upon the solution weight.
Following dilution, the pulp is separated from the solution and washed and thickened by well known methods. Optionally, the pulp is then acidified to a pH of between about 4 and 6.5, preferably between about 4.5 to 5.5, thickened and then formed into a web.
This acidification step has been found to significantly increase the brightness of the paper produced from the recovered pulp, and also avoids the necessity of bleaching the pulp.
The recovered stock may be blended with fresh virgin sulfate or sulfite stock, or with additional recovered stock to make cellulosic articles, such as newspaper and so forth.
In the de-inking process undesirable foam is encountered most commonly when high temperature and high agitation are experienced. It is also at this time that maximum detergency is needed to remove ink form the paper. Standard defoamers based upon ethylene bis-stearamide, silicone oils, or mineral oils while effective at the high temperatures and agitation conditions, become insoluble at lower temperatures and deposit on process equipment causing what is commonly referred to as pitch. Products of this invention have the defoaming properties of ethylene bis-stearamide, silicone oils, or mineral oils, but become soluble below their inverse cloud point.